1. Field of the Invention
This invention relates to an image reader for use in printing plate making apparatus and copying apparatus. More particularly, it relates to a method and apparatus for reading an image from a transparent original by synchronously translating a transmitting light source for emitting light toward the original and a reading scanner for receiving the light transmitted by the original, thereby scanning the original, characterized by providing synchronization in scanning speed or matching in scan start position between the transmitting light source and the reading scanner prior to scan reading of the original.
2. Prior Art
Advanced image readers for use in printing plate making apparatus and copying apparatus are required to have the ability to read not only reflective originals such as photographs and printed documents, but also transparent originals (or transparency) such as reversal film and negative film.
A conventional image reader includes a platen on which an original rests, a reading scanner disposed below the platen and an optical reading system coupled with the scanner. For reading a transparent original, there is added a transmitter assembly which is disposed on the platen for illuminating the original whereby the optical reading system receives the light transmitted by the original.
There are known a variety of transparency transmitter assemblies which are classified, depending on the manner of providing the light transmitted by the original, into the back light type using a tungsten halogen lamp or a similar lamp for illuminating the entire surface of the original, the shircusten type using a plurality of rod-shaped lamps for illuminating the entire surface of the original, and the light source scanning type using a rod-shaped lamp such as a fluorescent lamp with which the original is scanned in a vertical scan or auxiliary scan direction thereby providing two-dimensional scanning over the entire surface of the original.
The transparency transmitter assemblies of the back light and shircusten types use a simple mechanism, but require a great light intensity or quantity or a number of lamps and are difficult to illuminate the entire surface of a transparent original with a uniform light quantity (intensity). The transparency transmitter assembly of the light source scanning type is advantageously used in the application where image reading of high image quality is required as in printing plate formation.
The transparency transmitter assembly of the light source scanning type includes a transparent original transmitting light source unit having a rod-shaped light source for emitting reading light toward a transparent original, the reading light extending in the same one-dimensional direction as the light source in the reading scanner of the image reader. Also included is a drive source for translating the transmitting light source unit in an auxiliary scan direction or a direction substantially perpendicular to the one-dimensional direction.
The transparency transmitter assembly of the light source scanning type is disposed on the platen of the image reader and adapted to translate the original transmitting light source unit in the auxiliary scan direction for providing two-dimensional illumination to the transparent original on the platen. If the reading scanner of the image reader is translated synchronously with the transmitting light source unit, the light emitted by the rod-shaped lamp and transmitted by the transparent original is caught by the reading scanner and photoelectrically read by the optical reading system associated with the scanner.
In the transparency transmitter assembly of the light source scanning type, a transparent original is read by synchronously translating the light source unit of the transmitter assembly and the reading scanner of the image reader as described above. Accordingly, to ensure image reading of high quality with an optimum light quantity, the transmitting light source unit and the reading scanner must be synchronously translated for scanning with their optical axes aligned, that is, while maintaining them in a selected relative position.
The transparency transmitter assembly is used by setting it on the image reader in place only when it is desired to read a transparent original. The light source unit of the transmitter assembly and the reading scanner of the image reader have independent drive sources for their translation. Then even though both the components have an identical scanning speed at the stage of design, motors as the drive sources and transmission mechanisms for transmitting the motor drive force to the reading scanner or the transmitting light source unit inevitably have individual differences or mechanical errors which can introduce a difference in scanning speed between the transmitting light source unit and the reading scanner. It can also happen that the scanning speed is somewhat changed by repair or translation of the transmitting light source unit and the reading scanner. In such cases, the transmitting light source unit and the reading scanner cannot be maintained in a consistent relative position during the entire period of scan reading.
Also, even when the transmitting light source unit and the reading scanner have an identical scanning speed, if they cannot be brought to an adequate relative position for optical axis alignment at the initial when scan reading is started, then high-quality reading of a transparent original with an optimum light quantity is impossible.
During normal operation, the transparency transmitter assembly is not disposed on the image reader. When reading of a transparent original is desired, the user sets the transparency transmitter assembly in place on the image reader and hence, on the platen. The assembly includes a lid which is opened for replacement of the transparent original by a new one and closed on every reading operation.
On attachment of the transparency transmitter assembly and opening/closing of the lid thereof, the relative position between the transmitting light source unit and the reading scanner, especially between their base or home positions is changed. Then the transmitting light source unit and the reading scanner cannot be matched in position in the auxiliary scan direction at the initial when scan reading is started. With their optical axes misaligned, high-quality reading of a transparent original with an optimum light quantity is impossible.